The invention relates generally to electrical energy storage devices, and in particular to a hydrogen electrode in such devices.
Electrical energy can be stored in an electric battery. e.g. a lead-acid battery. In a time battery, the electrical energy is stored in the electrodes. Reduction-oxidation or redox reactions occur between the electrodes and an electrolyte in which the electrodes are immersed. As a result of the redox reactions, the electrodes are chemically changed during charging or discharging.
Another type of electrical energy storage device is a fuel cell. In a fuel cell reversible electrochemical reactions occur between the fuels. Typically the electrodes not only provide electrical contact to the electrochemical reaction but also catalyze the reaction. However, the electrical energy resides in the fuel and not in the electrodes. Thus the electrodes do not chemically change for a well functioning fuel cell. If the fuel cell is a closed system so that additional fuel is not introduced into the cell during its operation, such a fuel cell is often, if loosely, referred to as a battery because its operational characteristics are similar to those of a true battery.
A third type of electrical energy storage device is a hybrid of the previously described battery and fuel cell. One electrode reacts directly with an electrolyte, similarly to a battery electrode, while the other electrode only catalyzes a reaction between the electrolyte and another fuel, similarly to a fuel cell electrode. An example of such a hybrid is the nickel-hydrogen battery which is the subject of U.S. Pat. No. 3,867,119, issued to Dunlop et al.
A variant of the nickel-hydrogen cell of Dunlop will now be described with reference to the schematic cross-section of FIG. 1. A nickel electrode or cathode 10 is in contact with an electrolyte 12, such as potassium hydroxide (KOH), in aqueous solution. A redox reaction occurs between the nickel electrode 10 and the electrolyte 12. The reaction is of the form: EQU NiOOH+H.sub.2 O+e.sup.-.revreaction. Ni(OH).sub.2 +OH.sup.-.
The reaction flows to the right upon discharging and to the left upon charging.
On the other side of the cell there is a conductive anode 14 that is covered with a layer of platinum catalyst 16 in contact with another portion of the electrolyte 18. A semi-permeable membrane 20 allows hydrogen from a reservoir 22 to reach the anode 14 and the catalytic layer 16 but prevents the electrolyte 18 from flowing into the reservoir 22. The combination of the anode 14, the platinum catalyst 16 and the semi-permeable membrane 20 are commonly referred to as a negative electrode because they are usually formed in one structural unit. The reaction which occurs at the hydrogen electrode is of the form: EQU 1/2H.sub.2 +OH.sup.-.revreaction. H.sub.2 O+e.sup.-.
This reaction flows to the right upon discharging and to the left upon charging. As it is seen from the form of this reaction, the hydrogen is reacting with the electrolyte. The anode 14 serves only as an electrical conductor for the charge generated or consumed in the reaction and the platinum catalyst 16, while greatly promoting the reaction, is not consumed in the reaction.
The two portions of the electrolyte 12 and 18 are separated by a separator 24 which acts as an electrical barrier and for storage of the electrolyte 12 and 18.
The platinum catalyst 16 or electrocatalyst layer should have both hydrophilic and hydrophobic properties. It should be hydrophilic so that it admits the aqueous electrolyte and should be hydrophobic in order that it does not attract the electrolyte so strongly that the gaseous hydrogen cannot penetrate the catalyst 16. A conventional electrocatalyst layer 16 has an active component of platinum black, which is composed of high surface area platinum. The effect is to provide a very large surface area of the catalyzing platinum with a relatively small amount of expensive platinum. To provide the hydrophobic properties of the catalyst layer 16, the platinum black is dispersed in polytetrafluoroethylene (PTFE), marketed by Dupont under the trade name Teflon. The semi-permeable membrane 20, also referred to as the wet-proofing layer, is conventionally composed of a film of Teflon.
One prior art method of preparing a hydrogen electrode involves pasting the electrocatalyst onto a nickel screen substrate backed with a porous Teflon membrane. The procedure consists in preparing a slurry of platinum black, Teflon 30 emulsion and methocel (carboxymethyl cellulose). Nickel exmet (extruded metal) is formed by scoring regular slits in a nickel sheet and pulling the sheet transversely to the slits. The result is a perforated but continuous sheet of metal. An exmet is superior to a screen because of the absence of electrical contacts at the grid points. A Teflon backing is applied to the back of the nickel exmet and then the slurry is silkscreened onto the front of the nickel exmet. The electrode is then dried in air, heated to 100.degree. C. and finally sintered at 330.degree. C.
Another conventional procedure consists in spraying a mixture of platinum black, wetting agent and Teflon 30 emulsion onto a nickel exmet with a porous Teflon backing.
Most of the fabrication methods are directed at dispersing platinum black in Teflon so that a loading or density of platinum can be limited to 6-8 mg/cm.sup.2 of Pt. Recently, electrocatalyst formulations consisting of platinum dispersed in high surface area carbon have been invented which exhibit properties equivalent to that of platinum. The major benefit of using the combination of carbon and platinum is the reduction in cost.
Many U.S. patents have disclosed different methods of fabricating Teflon-bonded electrodes containing platinum. Solomon in U.S. Pat. No. 4,370,284 discloses a fabrication procedure for a non-bleeding electrode. A bleeding electrode is one that exhibits excessive hydrophobic tendencies so that bubbles of water form on its surface thus preventing the free flow of hydrogen gas. In Solomon's method, the electrocatalyst layer consists of 60-90% pure carbon and 10-40% polytetrafluoroethylene (PTFE). Furthermore a PTFE-containing wet-proofing layer is applied to the back of the electrode. The pore diameter in the electrocatalyst and the backing layer is controlled. A related defect is flooding in which an excessively hydrophilic electrode attracts too much electrolyte and prevents gas flow.
In U.S. Pat. No. 4,336,217, Sauer discloses a continuous manufacturing procedure for a plastic cohered gas diffusion electrode in which PTFE and carbon powder are mixed in a paddle mixer to enhance the degree of subdivision and homogeneity of the mixture in addition to temporarily vaporizing the PTFE. The formulation is then reduced to foils by means of powder rollers.
Baker et al., in U.S. Pat. No. 3,935,029, discloses an electrode fabrication procedure which uses a conductive carbon support produced by the rolling of carbon, a lubricant and PTFE.
Solomon, in another patent, U.S. Pat. No. 4,337,140, discloses an electrode containing an active layer of strengthened carbon and PTFE. The active layers are fibrillated along with a soluble pore-forming agent.
Witherspoon, in U.S. Pat. No. 3,779,872, discloses a gas electrode containing PTFE. The electrode is treated with magnesium acetate to increase the active surface area.
Baker et al, in U.S. Pat. No. 3,943,006, discloses an electrode structure for fuel cells in which catalysts, PTFE and sugar are blended together. The formulation is then rolled and the sugar is finally leached out.
Heffler, in U.S. Pat. No. 4,104,197, discloses a gas diffusion electrode having a hydrophobic layer containing pores with a diameter of 1.8 .mu.m and a hydrophilic layer containing pores of diameter 0.09 .mu.m. The electrode is prepared using a filter press.
All of the prior art electrodes suffer various disadvantages. In order to achieve proper electrolyte distribution among the positive and negative electrodes and the separator, the mean pore diameter of the negative electrode must be larger than the pore diameters of the other parts. This condition is not achieved in any of the prior art electrodes.
The hydrogen electrodes, based on pure platinum, are highly expensive. The less expensive hydrogen electrodes using pure carbon exhibit high polarization in a metal-hydrogen battery. Polarization is a measure of the efficiency of the hydrogen electrode. Under optimum conditions, the hydrogen electrode can produce a voltage of 0.9 V. However, if the reaction area is not sufficiently large for other reasons, this voltage may drop with the difference or polarization being a loss of energy and an inefficiency in voltage conversion.
Electrodes that are produced by pasting the electrocatalyst onto a mesh develop cracks and tears in the Teflon membrane in addition to spalling of the platinum black.